1. Field of the Invention
The present invention relates to the processing of paperboard blanks and, more particularly, to a method and apparatus for maintaining the proper registration between a rotary processing head and successive paperboard blanks.
2. Description of the Prior Art
During the manufacture of paperboard boxes, paperboard blanks are successively passed through a processing machine including a plurality of sections having cooperating rotary processing heads for performing operations on the paperboard blank. Such operations typically include, among others, printing graphics, forming crease lines and cutting each successive paperboard blank to thereby form a container blank which may be readily folded into a paperboard box.
More particularly, as the rotary processing heads of each section rotate in contact with successive paperboard blanks, each head performs an operation in a predetermined position on the blank. Therefore, each successive operation is superimposed on top of the preceding operation thereby forming the completed printed container blank. If the rotary processing heads of any section are not in proper registration or phase with the approaching paperboard blank to be operated upon, then the operation performed by the processing heads will not be correctly positioned on the paperboard blank. As such, the resulting superimposed operations are not properly aligned on the paperboard blank and the completed container blank is often rejected as an inferior product.
Prior art attempts to produce acceptable container blanks have included the strategy of using oversized blanks within the processing machine to account for registration errors between the processing heads and the paperboard blank. Operations which are misaligned on the paperboard blank due to registration errors are at least partially corrected by trimming excess material from either the leading edge or trailing edge of the oversized paperboard blank. While the resulting container blank is often acceptable for use in forming a paperboard box, this method of correcting for registration errors is expensive in terms of both time and wasted paperboard.
Additionally, the need to maintain accurate registration between rotary processing heads and paperboard blanks in the production of color printed container blanks has become more critical with higher production speeds and the demand for higher quality printing and multi-color graphics. In response, attempts have been made to provide paperboard processing devices which maintain the proper registration of each paperboard blank passing in contact with rotary processing heads.
Examples of typical registration devices may be seen in U.S. Pat. Nos. 4,618,391 and 5,383,392. However, such prior art devices have failed to provide reliable and consistent registration correction. The prior art devices fail to distinguish between registration errors which are correctable and those which are not correctable given the operating conditions and geometry of the processing machine. Additionally, the prior art devices fail to account for continuing and consistent registration errors as often caused by components within the processing machine. For example, operating members may impart drag forces on the paperboard blank or the paperboard blank may consistently slip relative to a forwardly conveying member.
Accordingly, there is a need for a method and apparatus for working paperboard blanks which maintains the accurate registration of a rotary processing head with successive paperboard blanks. Further, there is a need for such a method and apparatus which corrects the registration of the processing head relative to successive paperboard blanks while adjusting for recurring errors in registration between the processing head and the paperboard blank.